Field of the Invention
The present invention relates to a process for producing a nickel cobalt aluminum composite hydroxide and a process for producing a positive electrode active material for non-aqueous electrolyte secondary batteries. This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2014-221858 filed on Oct. 30, 2014 in Japan.
Description of Related Art
In recent years, there has been a strong demand for the development of compact and lightweight non-aqueous electrolyte secondary batteries having a high energy density due to the widespread use of portable electronic devices such as mobile phones and notebook computers. Further, there has also been a strong demand for the development of high-power secondary batteries as large-scale batteries such as power supplies for driving motors.
Examples of secondary batteries that satisfy these requirements include lithium ion secondary batteries. A lithium ion secondary battery includes a negative electrode, a positive electrode, and an electrolyte, and uses materials that can release and occlude lithium as a negative electrode active material and a positive electrode active material,
Lithium ion secondary batteries are now actively being researched and developed. Particularly, lithium ion secondary batteries using, as a positive electrode material, a layered or spinel-type lithium metal composite oxide can provide a 4 V-class high voltage, and are therefore practically used as batteries having a high energy density.
As a positive electrode material for such lithium ion secondary batteries, a lithium cobalt composite oxide (LiCoO2) that can be relatively easily synthesized is conventionally mainly used. However, attention is being given to a lithium nickel composite oxide (LiNiO2) that uses nickel cheaper than cobalt but is expected to have a higher capacity.
Meanwhile, a positive electrode material is required to include uniform particles having an appropriate particle diameter to allow a lithium ion secondary battery to achieve excellent performance (high cycle characteristic, low resistance, and high power).
This is because when a positive electrode material having a large particle diameter and a low specific surface area is used, an adequate reaction area between the positive electrode material and an electrolyte cannot be provided so that a high-power battery cannot be obtained due to an increase in reaction resistance, and on the other hand, when a positive electrode material having a wide particle size distribution is used, a problem occurs such as a reduction in battery capacity or an increase in reaction resistance. It is to be noted that the reason for the reduction in battery capacity is that a voltage is non-uniformly applied to the particles in an electrode so that fine particles are selectively degraded due to repeated charge and discharge.
Further, a shorter lithium ion migration distance between positive and negative electrodes is effective at increasing the output power of a lithium ion secondary battery. Therefore, a positive plate is required to be thinner. This is also the reason why positive electrode active material particles having a small particle diameter are useful.
Therefore, in order to enhance the performance of a positive electrode material, the above-described lithium nickel composite oxide also needs to be produced so as to have a uniform and small particle diameter.
Further, a lithium nickel composite oxide is usually produced from a composite hydroxide. Therefore, in order to produce a lithium nickel composite oxide including particles having a uniform and small particle diameter, a composite hydroxide used as a raw material thereof also needs to have a uniform and small particle diameter.
That is, in order to enhance the performance of a positive electrode material to produce a high-performance lithium ion secondary battery as a final product, a composite hydroxide used as a raw material of a lithium nickel composite oxide for forming a positive electrode material needs to include particles having a small particle diameter and a narrow particle size distribution.
As described above, a lithium nickel composite oxide is expected to have a high capacity, but has drawbacks such as a significant reduction in capacity after charge and discharge cycles and a low level of safety. It is known that addition of cobalt or aluminum is effective at overcoming such drawbacks. Particularly, a lithium nickel composite oxide containing 10 mol % or more of aluminum can significantly overcome such drawbacks.
For example, according to Patent Literature 1 that discloses a technique relating to an aluminum-containing lithium nickel composite oxide, a lithium nickel composite oxide is obtained by a solid-phase mixing method in which a nickel composite hydroxide and aluminum oxide or aluminum hydroxide are dry-mixed and calcined.                Patent Literature 1: JP 2008-130287 A        Patent Literature 2: Japanese Patent No. 4767484        Patent Literature 3: JP 2011-116608 A        
However, in the ease of the lithium nickel composite oxide obtained by a solid-phase mixing method in Patent Literature 1, segregation of aluminum is likely to occur even after calcination, which reduces the effect of improving a cycle characteristic and safety. In order to sufficiently exert such an effect in a lithium nickel composite oxide, it is important that aluminum be also coprecipitated during the production of a nickel composite hydroxide to uniformly distribute aluminum at the stage of a precursor.
Meanwhile, cobalt is similar in properties to nickel, and therefore it is relatively easy to homogeneously coprecipitate cobalt and nickel. However, aluminum is significantly different in properties from nickel, and therefore in the case of coprecipitation with nickel, aluminum hydroxide is particularly likely to precipitate singly due to an increase in aluminum content, which makes it difficult to obtain a composite hydroxide having a narrow particle size distribution.
For example, according to Patent Literature 2, a nickel cobalt aluminum composite hydroxide is produced as a precursor of LiNiCoAlO2 (lithium nickel cobalt aluminum composite oxide) with the use of nickel sulfate, cobalt sulfate, and aluminum sulfate as raw materials. However, according to the results of a study by the present inventors etc., when aluminum sulfate is used as an aluminum source, aluminum hydroxide precipitates at a lower pH than nickel hydroxide or cobalt hydroxide, and is therefore likely to precipitate singly, which makes it impossible to obtain a composite hydroxide having a narrow particle size distribution.
According to Patent Literature 3, a nickel cobalt aluminum composite hydroxide is produced as a precursor of a lithium nickel composite oxide with the use of sodium aluminate as an aluminum source. According to the results of a study by the present inventors etc., the use of sodium aluminate as an aluminum source allows aluminum hydroxide to precipitate at a pH close to a pH at which nickel hydroxide or cobalt hydroxide precipitates, which makes it easier to cause coprecipitation than when aluminum sulfate is used. However, an aqueous sodium aluminate solution is unstable, and therefore when sodium aluminate is used singly (when sodium and aluminum are equal in molar quantity), aluminum hydroxide does not stably precipitate, which makes it difficult to stably obtain a composite hydroxide having a uniform particle diameter.
As described above, a nickel cobalt aluminum composite hydroxide as a precursor of a positive electrode active material for non-aqueous electrolyte secondary batteries, which includes a lithium nickel composite oxide having an excellent cycle characteristic, a high level of safety, and a highly-uniform and small particle diameter, has not yet been stably supplied, and therefore there has been a demand for the development of a process for producing such a composite hydroxide.
In light of the above problems, it is an object of the present invention to provide a process for producing a nickel cobalt aluminum composite hydroxide having an excellent cycle characteristic, a high level of safety, and a highly-uniform and small particle diameter, and a process for producing a positive electrode active material for non-aqueous electrolyte secondary batteries.